Biology
During a long evolution, many pathogenic microorganisms have learned to exploit eukaryotic cell surface glycoconjugates, i.e. glycolipids, glycoproteins and proteoglycans, as receptor molecules for cell attachment to facilitate tissue colonization and invasion processes. In brief, specific proteins called adhesins of the surface of bacteria, viruses, fungi and parasites interact with carbohydrate chains of glycoconjugates which enable microbes to colonize mucosal surfaces and tissue lesions.
The role of sialic acid in binding of pathogens to host cells has been reported over many years. Only recently proteoglycans with their carbohydrate chains (glycosaminoglycans) were shown to bind many different pathogens. By removing terminal carbohydrate moieties of these various glycoconjugates with sialidase and other exoglycosidases or with glycosaminoglycan (GAG) degrading enzymes on the cells in monolayers, these structures were proven to be receptor molecules for various sialoadhesins and heparan sulfate binding proteins (HeBPs).
These mechanisms are summarized in a review article by Siiri Hirmo, Meeme Utt and Torkel Wadström, Biology, Biochemistry, Clinical Biochemistry, Volume 12, including Proceedings from the 17th International Lectin Meeting in Wütrzburg, 1997, edited by Edilbert van Driessche, Sonia Beeckmans and Thorkild C. Bøg-Hansen, published by TEXTOP, Lemchesvej 11, DK-2900 Hellerup, Denmark, ISBN number 87-984583-0-2.
During microbial infections, inflammatory mediators are released and activated. These so-called “pro-inflammatory cytokines” include tumor necrosis factor alpha and beta (TNF-α and TNF-β), interleukin-1 (IL-1), and interleukin-6 (IL-6). These cytokines are part of the inflammatory response of sepsis. Multiple organ failure induced by sepsis is currently the leading cause of death in intensive care units.
In connection with microbial infections and cardiovascular surgery, for instance cardiopulmonary bypass, inflammatory responses are elicited and have a multitude of biological consequences, ranging from subclinical organ dysfunction to severe multiorgan failure. Cytokines are thought to be important mediators in this response.
The cytokines mentioned above have a capacity to bind selectively to a range of glycosaminoglycans, or GAGs, including heparan sulfate in tissues and on the surface of both endothelial cells and leucocytes.
Receptors
Heparan sulfate is a glycosaminoglycan that is present on the surface of almost all mammalian cells. It is built up by alternating D-glucosamine and uronic acid residues (L-iduronic and D-glucuronic). Heparan sulfates are highly charged (sulfated) heterogeneous polysaccharides and represent the carbohydrate portion of many glycoconjugates (syndecan, perlecan, glypican) on the cell surface.
Many microbes utilize heparan sulfates on the surface of the mammalian cell as receptors. This mechanism is general and valid for almost all bacteria, virus and parasites. Some microorganisms utilize more than one glycoconjugate receptor. Examples of other receptors that are used together with heparan sulfate are specific chondroitin sulfates and sialic acid containing glycoproteins.
Heparan sulfate/chondroitin sulfate binding microbes are exemplified by viruses like herpes simplex virus type 1 (HSV-1), causative agent of orolabial herpes; herpes simplex virus type 2 (HSV-2), causative agent of genital herpes; cytomegalovirus (CMV), the major complicating agent in immunosuppressed patients; dengue virus, which causes recurrent fevers; and human immunodeficiency virus (HIV); and by bacteria like Helicobacter pylori, Streptococcus sanguis, Streptococcus mutans, Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and Mycobacterium tuberculosis; and parasites like Plasmodium falciparum (which causes malaria), and Trypanosoma cruzi (which causes trypanosomiasis).
Further, cytokines, like TNF-β, also utilize heparan sulfate on cell surfaces for binding and activation.
Heparin as a Receptor
Heparin is a polysaccharide, which is isolated from mammalian tissue. Since its discovery in 1916 by the American scientist McLean, heparin has been recognized for its blood anticoagulant properties and heparin has, for more than 50 years, been used clinically as a blood anticoagulant and antithrombotic agent.
Whereas heparan sulfates are ubiquitous components of all tissue-organized animal life forms, heparin has a very particular distribution in mammalian tissue. Heparin is, in contrast to the heparan sulfates, present only in the basophilic granules of mast cells. However, today, in addition to its established place in prevention and therapy of thromboembolic disorders, heparin has demonstrated a broad spectrum of different activities independent of anticoagulation.
A large number of proteins in blood bind, with high affinity, to heparin and/or heparan sulfate. Examples are antithrombin (AT), fibronectin, vitronectin, growth factors (e.g. the fibroblast growth factors, the insulin like growth factors etc). Human serum albumin (HSA) also binds, but with a lower affinity. On the other hand, HSA is present in large amounts in blood.
To utilize these properties of heparin for hindering infections, introducing heparin fragments and/or sialic containing fragments into the vascular system has been contemplated. Thereby, it was thought, these fragments would bind to the lectins on the microbes, block them and thus hinder them from binding to the receptors on the mammalian cell surface. This concept has been tried by many scientists but with limited success, in most cases due to bleeding complications when large amounts of heparin are introduced into the vascular system.
U.S. Pat. No. 6,197,568 dicloses methods for isolation and detection of flaviviruses and other hemorrhagic fever viruses, such as dengue virus, based on the sulfated polyanion-dependent interaction of flaviviruses and hemorrhagic fever viruses.
Extracorporeal devices are used in a variety of clinical situations including kidney dialysis, cardiopulmonary bypass and plasmapheresis. “Extracorporeal therapies” means procedures in which desired products like oxygen, blood-anticoagulants, anesthetics etc can be added to body fluids. Conversely, undesired products like toxins etc can be removed from body fluids outside the body. Examples are haemodialysis and haemofiltration which represent technologies whereby blood is rinsed from waste products.